(a) Field of the Invention
The present invention relates to an optical fiber having a lower bending loss suitable for wavelength division multiplexing (WDM) transmission. The present invention also relates to an optical fiber cable including such an optical fiber.
(b) Description of the Related Art
Along with the development of higher bit-rate and higher capacity optical transmission, the WDM transmission technique attracts larger attentions in the optical transmission field. In the current WDM transmission field, there are a request for further increasing the transmission capacity, and an attempt for enlarging the wavelength range for the optical transmission.
It is noted in the conventional optical fiber that the optical fiber is likely to include as mixed impurities the OH group having an absorption peak around a wavelength of 1383 nm. Thus, in general, the conventional optical fiber is not suitable for the WDM transmission around a wavelength of 1383 nm.
In order to solve the above problem, Patent Publication U.S. Pat. No. 6,205,268 discloses a specific optical fiber having a refractive index profile similar to that of a typical single mode optical fiber (referred to as SMF hereinafter) having a zero-dispersion wavelength in a wavelength band of 1.31 μm and yet having a reduced absorption loss caused by the OH group. The disclosed optical fiber is designed to have a wide wavelength range between 1285 nm and 1600 nm in the WDM transmission.
A typical optical fiber generally comprises a core region disposed at the central area of the fiber and a cladding surrounding the outer periphery of the core, and at least two coating layers surrounding the outer periphery of the cladding. The coating layers have different Young's moduli and are made of ultraviolet-cured resin or thermo-cured resin.
FIG. 8 shows an optical fiber ribbon, generally designated by numeral 20, including a plurality of optical fibers 10. The optical fibers 10 each coated with a colored layer 15 on the outer periphery thereof are arranged in a plane to form a parallel array. The optical fibers 10 are covered with ultraviolet-cured resin all together.
An optical fiber cable is widely used which includes therein a plurality of such optical fiber ribbons each having, for example, four, eight, twelve, or twenty-four optical fibers.
The optical fiber cable used in a metropolitan area has a larger number of optical fibers as much as about 1000. FIG. 9 shows a cross sectional view of a slotted core type optical fiber ribbon cable.
The slotted core type optical fiber ribbon cable 30 shown in FIG. 9 has a slot rod 31 having in the exterior surface thereof a plurality of slots 32 each having a helical pitch. Each slot 32 accommodates therein a plurality of optical fiber ribbons 20 stacked as shown in FIG. 8. Each optical fiber ribbon 20 may include four, eight or more optical fibers.
The slot rod 31 is an elongated plastic member made of polyethylene, and includes at the center thereof a strength member 33 such as a metallic wire or a fiber-reinforced plastic (FRP) rod. The outer periphery of the slot rod 31 is wrapped with a tape 34, and then covered with a plastic sheath 35 such as polyethylene or polyvinyl chloride.
The slotted core type optical fiber ribbon cable 30 as described above has the advantage of higher-density integration of optical fibers and higher reliability in the mechanical characteristics thereof.
The slotted core type optical fiber ribbon cable 30 is generally free from disarrangement of the optical fiber ribbons due to the structure thereof. However, when bending is excessively applied to the optical fiber cable, the ends of the stacked optical fiber ribbons 20 may be pushed against the inner wall of the slot 32 and the transmission loss of the optical fibers in the optical fiber ribbons may increase, due to a lateral force therefrom.
Especially, four fibers at the corners of the stacked optical fiber ribbons tend to show higher increase of the transmission loss.
The specific optical fiber described in the patent publication U.S. Pat. No. 6,205,268 has a refractive index profile similar to that of the SMF and has a reduced absorption loss around 1383 nm caused by the OH group, but generally has a higher bending loss compared to a dispersion shifted fiber (DSF) having a higher refractive index at the core region thereof.
Therefore, after accommodated into the slotted core type optical fiber ribbon cable, the specific optical fiber shows higher increase of the transmission loss, due to the lateral force. The higher increase of the transmission loss is remarkable especially in the L-band, wavelength band between 1565 nm and 1625 nm, and may exceed 1 dB/km, which was a problem for applying the slotted core type optical fiber ribbon cable using the specific optical fiber to a wide wavelength range of WDM optical transmission.